Production of sound

Vibration means a kind of rapid to and fro motion of an object. The sound of the human voice is produced due to vibrations in the vocal cords.

Propagation of Sound

• Sound is produced by vibrating objects. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas.

• Sound moves through a medium from the point of generation to the listener.

• When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from the vibrating object to the ear. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. As a result of which the adjacent particle gets displaced from its position of rest. After displacing the adjacent particle the first particle comes back to its original position. This process continues in the medium till the sound reaches your ear.

• The disturbance created by a source of sound in the medium travels through the medium and not the particles of the medium.

Wave

• A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion.n. They in turn produce similar motion in others

• The particles of the medium do not move forward themselves, but the disturbance is carried forward. This is what happens during propagation of sound in a medium, hence sound can be visualised as a wave

• Sound waves are characterised by the motion of particles in the medium and are called mechanical waves.

• Air is the most common medium through which sound travels. When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C) .This compression starts to move away from the vibrating object.

• When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).

• As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave that propagates through the medium.

• Compression is the region of high pressure and rarefaction is the region of low pressure.

• Pressure is related to the number of particles of a medium in a given volume.

• More density of the particles in the medium gives more pressure and vice versa.

• Thus, propagation of sound can be visualised as propagation of density variations or pressure variations in the medium

Sound needs a medium to travel

Sound is a mechanical wave and needs a material medium like air, water, steel etc. for its propagation. It cannot travel through vacuum.SOUND WAVES ARE LONGITUDINAL WAVES

Longitudinal waves

A longitudinal wave is the one in which the individual particles of the medium move about their mean positions in a direction parallel to the direction of wave propagation.

Sound is an example of a longitudinal wave. A compression is where the particles of the medium are closest together, and a rarefaction is where the particles are farthest apart.

Transverse waves

• There is also another type of wave, called a transverse wave. In a transverse wave particles do not oscillate along the direction of wave propagation but oscillate up and down about their mean position as the wave travels.

• When we drop a pebble in a pond, the waves you see on the water surface is an example of transverse wave.

• Light is a transverse wave but for light, the oscillations are not of the medium particles or their pressure or density – it is not a mechanical wave.

A transverse wave is the one in which the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation.

Characteristics of a Sound Wave

• Wavelength-The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength, The wavelength is usually represented by λ.Its SI unit is metre (m).

• Frequency-. The number of oscillations of the particles of sound wave per unit time is the frequency of the sound wave.If we can count the number of the compressions or rarefactions that cross us per unit time, we will get the frequency of the sound wave. It is usually represented by ν.Its SI unit is hertz (symbol, Hz).

• Time period-The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave..In other words, the time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave.The letter T is used to symbolise it. Second is its SI unit (s).Frequency and time period are related as follows: F=1/T where F is frequency and T is time period.

• Amplitude-The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A.For sound its unit will be that of density or pressure.

• Intensity-The amount of sound energy passing each second through unit area is called the intensity of sound.

Important points to remember

• A violin and a flute may both be played at the same time in an orchestra. Both sounds travel through the same medium, that is, air and arrive at our ear at the same time. Both sounds travel at the same speed irrespective of the source. But the sounds we receive are different. This is due to the different characteristics associated with the sound. Pitch is one of the characteristics. How the brain interprets the frequency of an emitted sound is called its pitch. The faster the vibration of the source, the higher is the frequency and the higher is the pitch, Thus, a high pitch sound corresponds to more number of compressions and rarefactions passing a fixed point per unit time. Objects of different sizes and conditions vibrate at different frequencies to produce sounds of different pitch.

• The loudness or softness of a sound is determined basically by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate. If we strike a table lightly, we hear a soft sound because we produce a sound wave of less energy (amplitude). If we hit the table hard we hear a louder sound. A sound wave spreads out from its source. As it moves away from the source its amplitude as well as its loudness decreases. Louder sound can travel a larger distance as it is associated with higher energy

• The quality or timber of sound is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness. The sound which is more pleasant is said to be of a rich quality. A sound of single frequency is called a tone.

• The sound which is produced due to a mixture of several frequencies is called a note and is pleasant to listen to. Noise is unpleasant to the ear! Music is pleasant to hear and is of rich quality.

Speed of sound

• The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time. It is also called wave velociy

• Speed = wavelength × frequency.

• The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.

SPEED OF SOUND IN DIFFERENT MEDIA

Sound propagates through a medium at a finite speed. The sound of a thunder is heard a little later than the flash of light is seen. So, we can make out that sound travels with a speed which is much less than the speed of light.

The speed of sound depends on the properties of the medium through which it travels,which are

• Characteristics of the material medium through which the wave travels

• Density.

• Temperature.

• Humidity.

Reflection of Sound

• The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane.

• An obstacle of large size which may be polished or rough is needed for the reflection of sound waves.

ECHO

An echo is a sound produced when sound waves are reflected off of a surface and returned to the listener. It is the sound that has been reflected, and it reaches the listener after the original sound has passed.

Conditions for Echo

• The sensation of sound persists in our brain for about 0.1 s. To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s.

• For hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be 17.2 m.

• This distance will change with the temperature of air.

Echoes may be heard more than once due to successive or multiple reflections. The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land.

REVERBERATION

• A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation

• In an auditorium or big hall excessive reverberation is highly undesirable. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

USES OF MULTIPLE REFLECTION OF SOUND

• Megaphones or loudhailers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions.In these instruments, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward direction towards the audience

• A stethoscope is a medical device used to hear sounds made by the body, primarily in the heart or lungs. The doctor's ears receive several reflections of the patient's heartbeat through stethoscopes.

• Generally the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall. Sometimes a curved soundboard may be placed behind the stage so that the sound, after reflecting from the sound board, spreads evenly across the width of the hall.

Range of Hearing

The audible range of sound for human beings extends from about 20 Hz to 20000 Hz. Children under the age of five and some animals, such as dogs can hear up to 25 kHz (1 kHz = 1000 Hz). As people grow older their ears become less sensitive to higher frequencies.

Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound

• Rhinoceroses communicate using infrasound of frequency as low as 5 Hz

• It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves begin which possibly alert the animals.

Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.

• Ultrasound is produced by animals such as dolphins, bats and porpoises.

• Moths of certain families have very sensitive hearing equipment. These moths can hear the high frequency squeaks of the bat and know when a bat is flying nearby, and are able to escape capture.

• Rats also play games by producing ultrasound.

Applications of Ultrasound

Ultrasounds are high frequency waves. Ultrasounds are able to travel along well defined paths even in the presence of obstacles. Ultrasounds are used extensively in industries and for medical purposes.

• In general, ultrasound is used to clean parts that are in difficult-to-reach regions, such as spiral tubes, odd-shaped parts, electronic components, etc. Objects to be cleaned are placed in a cleaning solution and ultrasonic waves are sent into the solution. The dust, grease, and filth particles separate and fall out as a result of the high frequency. Thus, the items receive complete cleaning.

• Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called ‘echocardiography’

• Ultrasound may be employed to break small ‘stones’ formed in the kidneys into fine grains. These grains later get flushed out with urine.

• Ultrasonography is a procedure that uses ultrasound to look at tissues and organs inside the body. The sound waves make echoes that form pictures of the tissues and organs on a computer screen (sonogram). Ultrasonography may be used to help diagnose diseases, such as cancer.

SONAR

The acronym SONAR stands for SOund Navigation And Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.

• Sonar consists of a transmitter and a detector and is installed in a boat or a ship

• The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector

• The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted.

• The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound.

• The total distance, 2d travelled by the ultrasound is, 2d = v × t

• The above method is called echo-ranging. The sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs, sunken ship etc.